Chlorine, atom/group transfer reactions

Starnes et al.hl have also suggested that the head adduct may undergo p-scission to eliminate a chlorine atom which in turn adds VC to initiate a new polymer chain. Kinetic data suggest that the chlorine atom does not have discrete existence. This addition-elimination process is proposed to he the principal mechanism for transfer to monomer during VC polymerization and it accounts for the reaction being much more important than in other polymerizations. The reaction gives rise to terminal chloroallyl and 1,2-dichlorocthyl groups as shown in Scheme 4.8. 

The symmetric series provides functional cyclohexadienes, whereas the non-symmetric one serves to build deuterated and/or functional arenes and tentacled compounds. In both series, several oxidation states can be used as precursors and provide different types of activation. The complexes bearing a number of valence, electrons over 18 react primarily by electron-transfer (ET). The ability of the sandwich structure to stabilize several oxidation states [21] also allows us to use them as ET reagents in stoichiometric and catalytic ET processes [18, 21, 22]. The last well-developed type of reactions is the nucleophilic substitution of one or two chlorine atoms in the FeCp+ complexes of mono- and o-dichlorobenzene. This chemistry is at least as rich as with the Cr(CO)3 activating group and more facile since FeCp+ activator is stronger than Cr(CO) 3. 

Entries 7 and 8 illustrate conversion of diazonium salts to phenols. Entries 9 and 10 use the traditional conditions for the Sandmeyer reaction. Entry 11 is a Sandmeyer reaction under in situ diazotization conditions, whereas Entry 12 involves halogen atom transfer from solvent. Entry 13 is an example of formation of an aryl iodide. Entries 14 and 15 are Schiemann reactions. The reaction in Entry 16 was used to introduce a chlorine substituent on vancomycin. Of several procedures investigated, the CuCl-CuCl2 catalysis of chlorine atom transfer form CC14 proved to be the best. The diazonium salt was isolated as the tetrafluoroborate after in situ diazotization. Entries 17 and 18 show procedures for introducing cyano and azido groups, respectively. 

Instability of the polymer is responsible for the primary step in decomposition and is attributed either to fragments of initiator or to branched chains or to terminal double bonds. The appearance of branching is the result of reactions of chain transfer through the polymer, while that of unsaturated terminal groups results from reaction of disproportionation and chain transfer through the monomer. During thermal and thermo-oxidative dehydrochlorination of PVC, allyl activation of the chlorine atoms next to the double bonds occurs. In this volume, Klemchuk describes the kinetics of PVC degradation based on experiments with allylic chloride as a model substance. He observed that thermal stabilizers replace the allylic chlorine at a faster ratio than the decomposition rate of the allylic chloride. 

When metal ion complexed amino radicals are produced by the reaction of A -chloro amines with reducing metal salts in the presence of alkenes, /6-halo amines are produced12-39 41. The reaction of 1-chloropiperidine with cyclohexene, iron(II) sulfate and iron(III) chloride in methanol afforded mainly the d.s-adduct of 2. The diastereoselectivity is attributed to coordination of the unprotonated amino group with the iron(III) salt, which is mainly responsible for the chlorine atom transfer. With A-chlorodimethylarnine and 4-chloromorpholine lower yields are obtained. 

Just like the ability to bind HCl, this exchange reaction is a general characteristic of all efficient PVC heat stabilizers and stabilizer systems. An essential condition of this exchange reaction, is of course, that the transferred groups - in this case a mercaptocarbonic acid ester group - have a lower tendency to be eliminated than the chlorine atom. 

Work in the group of Speckamp has shown that C-Cl bonds in a captodative position are weak enough to lead to radical chain cyclization reactions by chlorine atom transfer [28], Chlorine atom transfer from 34 to the catalyst, Cu Cl-bipyridine, leads to radical 35 which then undergoes 5-exo intramolecular addition to form the proline derivative 36 (Eq. 1). The captodative substitution is necessary for this radical process in the absence of an electron-withdrawing substituent, a cationic reaction leading to a piperidine occurs instead [29]. 

Another important pioneering work in CyD chemistry was reported by Breslow and Campbell [7] on the chlorination of anisole with HOCl. In the presence of a-CyD, the chlorination occurs exclusively at the para-position of anisole. This reaction also takes advantage of covalent catalysis. Here, HOCl first reacts with the secondary OH group of CyD, and the chlorine atom is selectively transferred to the para-position of anisole. Since the anisole penetrates into the cavity with the methoxy group first, the para-position of anisole is located near the secondary OH group (and thus near the CyD-OCl group). Apparently, the product selectivity comes from a proximity effect , as is often observed in enzymatic reactions. 

Chloromethylated polystyrene in both crosslinked and linear form, is even more amenable than polystyrene to chemical transformation. Chloromethylation is a reaction that is best avoided and the simplest way of incorporating these functional groups into polymers is by copolymerization with vinylbenzyl chloride, which is now becoming more readily available. The chlorine atom is fairly easily displaced by nucleophilic reagents and several reactions of this kind promoted by phase-transfer catalysts have been investigated by Nishikubo et a/. (Scheme 8). The... 

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